Investigation on the Durability of E-Glass/Epoxy Composite Exposed to Seawater at Elevated Temperature

Article Investigation on the Durability of E-Glass/Epoxy Composite Exposed to Seawater at Elevated Temperature

Amir Hussain Idrisi 1, Abdel-Hamid I. Mourad 1,2,3,* , Beckry M. Abdel-Magid 4 and B. Shivamurty 5

1 Department of Mechanical Engineering, United Arab Emirates University, Al Ain 15551, United Arab Emirates; [email protected] 2 National Water and Energy Center, United Arab Emirates University, Al Ain 15551, United Arab Emirates 3 Mechanical Design Department, Faculty of Engineering, Helwan University, Cairo 11795, Egypt 4 Department of Composite Materials Engineering, Winona State University, Winona, MN 5598, USA; [email protected] 5 Department of Mechanical and Manufacturing Engineering, Manipal Institute of Technology, Manipal 572104, India; [email protected] * Correspondence: [email protected]

Abstract: In this manuscript, the durability of the E-glass/epoxy composite was determined under a seawater environment. The effect of harsh environment was investigated in terms of seawater absorption, microstructure and degradation in mechanical properties. E-glass epoxy composite specimens were conditioned in gulf seawater at 23 ◦C, 65 ◦C and 90 ◦C for the period of 12 months. It was observed that the mass of the samples increased after the immersion of 12 months at 23 ◦C and 65 ◦C whereas it reduced at 90 ◦C. The salt deposition was observed at the surface of specimens ◦ ◦ without any crack for the seawater conditioning at 23 C and 65 C. The swelling and crack formation were significantly visible on the surface of the specimen immersed for 12 months at 90 ◦C. It indicates Citation: Idrisi, A.H.; Mourad, that the degradation mechanism accelerated at elevated temperature results fiber/matrix debonding. A.-H.I.; Abdel-Magid, B.M.; The tensile test indicates slight variation in the elastic modulus and reduction in strength of E-glass Shivamurty, B. Investigation on the epoxy composite by 1% and 9% for specimens immersed at 23 ◦C and 65 ◦C respectively. However, Durability of E-Glass/Epoxy at 90 ◦C, the tensile strength sharply decreased to 7% and elastic modulus significantly increased in Composite Exposed to Seawater at the exposure of 12 months. A prediction approach based on a time-shift factor (TSF) was used. This Elevated Temperature. Polymers 2021, 13, 2182. https://doi.org/ model predicted that the strength retention of E-glass/Epoxy composite will be reduced to 7% in ◦ 10.3390/polym13132182 450 years after immersion in seawater at 23 C. Lastly, the activation energy for the degradation of the composite was calculated. Academic Editor: Youhong Tang Keywords: glass fiber composite; elevated temperature; mechanical properties; microstructural Received: 13 June 2021 analysis; durability; modeling Accepted: 27 June 2021 Published: 30 June 2021

Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in In the past few years, several researchers have investigated the characteristics of com- published maps and institutional affil- posites in the marine environment. Glass Fiber-Reinforced Polymer (GFRP) composites iations. are commonly used in automobile, aerospace and marine industries because of their high strength to weight ratio, stiffness and durability. Furthermore, glass fibers are simple to manufacture, economical, less fragile and high chemical resistant compared to carbon fiber. However, GFRPs face challenges in various environmental surroundings such as UV, sea- Copyright: © 2021 by the authors. water, alkaline and various other corrosive environmental conditions [1–8]. Polymer matrix Licensee MDPI, Basel, Switzerland. composites trap water in voids at high temperatures and are often fused into hydroxyl This article is an open access article radicals of the epoxy polymers [9–12]. The immersion in water results in deterioration in distributed under the terms and the physical properties of composites [13–18] which makes epoxy polymers inflate, breaks conditions of the Creative Commons their bonds due to hydrolysis and further plasticizes [19–21]. The hot water environment Attribution (CC BY) license (https:// contributes to small cracks within the polymer matrix due to water absorption and osmotic creativecommons.org/licenses/by/ edge rupture [22–24], which increases the degradation of composites with immersion time. 4.0/).

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The durability of the structures also affected by the presence of defects on the surface of the polymer matrix composite before immersion. It can be regulated by determining the effect of the critical size of the osmotic blister [25]. Furthermore, the strength of the structure can be improved by repairing the structures with glass fiber-reinforced polymer [26,27]. For material effectiveness, it is essential to ensure the durability of FRP composites in the seawater environment. Nanofillers are one effective solution for closing any gaps and increasing the compositional intensity [28–40]. Several studies have been performed in different aging conditions for the performance of epoxy-based composites. It was previously observed that the flexural properties of FRP samples decreased due to seawater immersion [41,42]. Pavan et al. [43] investigated that tensile strength of glass/epoxy reduced from 200.83 MPa to 146.42 MPa, and 185.27 MPa for atmospheric and subzero temperatures aging for 3600 h. Yan et. al. [44] noticed a similar reduction in the mechanical properties of flax-fabric/epoxy composites. The tensile strength of the specimens reduced by 31.1%, 28.3% and 22.6% under the immersion of alkaline (5% NaOH) solution, seawater and water respectively. Silva et al. [45] studied the durability of GFRP laminates with epoxy resin by exposing the composites to saltwater at 30 ◦C, 50 ◦C and 65 ◦C for 5000 h. For the first 1500 h, the modulus behavior indicated that the prevailing damage mechanism was swelling. Then, between 1500 h and 2500 h, the elastic modulus behavior represented that plasticization was the predominant damage mechanism, especially for samples aged at 30 ◦C. Chakraverty et al. [46] studied the effect of seawater conditioning for up to 12 months (m) on the mechanical properties, such as interlaminar shear strength (ILSS), tensile stress and failure strain, elastic modulus and glass transition temperature (Tg) of GFRP composites. The tensile stress was reduced to 79% after immersion for 6 months but a slow recovery was observed, and the tensile strength rebounded to 84% after immersion for 12 months. Strain at rupture decreased by 12% and 20% after 6 months and 12 months of seawater immersion respectively. These reductions in values of stress and strain at rupture are due to the plasticization and swelling in the composite material. Chen Y. et al. [47] utilized the Arrhenius equation-based prediction model for predicting the durability of GFRP-reinforced concrete structures. The GFRP bars were immersed in a concrete solution at temperatures of 20 ◦C, 40 ◦C, and 60 ◦C. The short-term data were used for the accelerated aging tests. Similarly, Ali et al. [48] investigated the durability performance of the BFRP bars. The specimens were conditioned at 60 ◦C after different immersion periods (1000 h, 3000 h, and 5000 h). The prediction results show that the shear strength of BFRP bars will reduce by 19.8% and 23.0% after 150 years of immersion in the alkaline solution at 10 ◦C and 30 ◦C respectively. Dejke [49] investigated the durability of GFRP bars in alkaline solutions immersed at 20 ◦C, 40 ◦C, 60 ◦C and 80 ◦C, for a duration of 568 days. Samples were tested for tensile strength after removal from the solution. The time shift factor (TSF) approach was used for the prediction of tensile strength retention. It was observed that samples immersed at 60 ◦C for 1.5 years have the same tensile strength retention as for 50 years at 7 ◦C. Table1 summarizes the research related to E-glass/epoxy composite immersion at different temperatures. It is clear from the above literature review and research relevant to this topic mentioned in Table1 that the data for harsh environment conditions (i.e., conditioning at 90 ◦C) are very lacking. Additionally, the harsh environmental conditioning is limited to a small exposure time. To the authors’ best knowledge, the durability evaluation of the fiber-reinforced polymer composites at a high temperature for one year has not been discussed in the literature. The main objective of this work is to conduct an experimental investigation to evaluate the durability of E-glass/epoxy composite in harsh environmental conditions. Specimens were conditioned at different exposure temperatures (23 ◦C, 65 ◦C and 90 ◦C) in Gulf seawater for a period of 12 months. After exposure to the harsh environment, tensile strength, tensile strain and Young’s modulus of the specimen were determined. Furthermore, water absorption and scanning electron microscopy (SEM) analysis was conducted to investigate the damage mechanism of the composite. In addition to determining the degradation of Polymers 2021, 13, 2182 3 of 19

the composite experimentally, these data were used to predict the durability of composites using Arrhenius-based models and the time-shift factor (TSF) approach.

Table 1. FRP composite immersed at different environmental conditions.

Authors Composite Conditioning Duration Silva et al. [45] E-glass/epoxy Saltwater at 30 ◦C, 45 ◦C and 55 ◦C 750–5000 h (Approx 7 months) Chakraverty et al. [46] E-glass/epoxy Seawater at room temperature 2, 4, 6, 8, 10, and12 months (1 year) Glass/polydicycl-opentadiene Saltwater and deionized water Hu et al. [50] 1, 3, 6, and 12 months (1 year) and glass/epoxy at·60 ◦C Glass/epoxy, carbon/epoxy and Guermazi et al. [51] Tap water at 24 ± 3, 70 and 90 ◦C 3 months glass/carbon/epoxy Bobba et al. [52] E-glass and S-glass fiber-epoxy Tap water at 90 ◦C 600, 1200, and 1800 h (2.5 months) H SO , NaOH and NaCl at 60 Feng et al. [53] Glass/epoxy 2 4 7, 15, 30, and 90 days (3 months) and 90 ◦C Glass fiber-reinforced epoxy Merah et al. [54] Seawater at outdoor temperature 6 and 12 months (GFRE) Glass/epoxy and Mourad et al. [18] Seawater at 23 ◦C and 65 ◦C 3, 6, 9, and 12 months glass/polyurethane EminDeniz et al. [55] Glass/epoxy composite Seawater at 20 ◦C 3, 6, 9, and 12 months Artificial seawater in sub-zero and 3600 h Pavan et al. [43] E-glass/epoxy laminates ambient temperatures (5 months) Basalt fiber-reinforced plastic Wei et al. [56] (BFRP) and Glass fiber-reinforced Artificial seawater at 25 ◦C 10, 20, 30, 60, and 90 days plastic (GFRP) Glass/epoxy filament Antunes et al. [57] Seawater at 80 ◦C 7–28 days wound cylinders Artificial seawater at room Ghabezi et al. [58] Carbon/epoxy and glass/epoxy 45 days temperature and 60 ◦C

2. Experimentation In this study, E-glass/epoxy composite reinforced with 52 vol% of glass ﬁber was utilized. The E-glass/epoxy was manufactured through a continuous lamination process Polymers 2021, 13, x FOR PEER REVIEW and obtained from Gordon Composites, Inc. Samples with thickness4 of of 3 21 mm for the tensile test were prepared as per ASTM Standard D-3039 [59]. Samples were conditioned at different exposure temperatures (23 ◦C, 65 ◦C and 90 ◦C) in Gulf seawater for the period of 12 months, as shown in Figure1. Specimens conditioned at 23 ◦C and 65 ◦C were tested after every three monthsafter every whereas three months the testin whereasg frequency the testing was frequency one month was for one the month specimen for the specimen immersed at 90 °C.immersed at 90 ◦C.

(a) (b) (c)

Figure 1. ConditioningFigure of 1. Conditioningspecimen at of(a) specimen 23 °C, (b at) 65 (a) °C 23 ◦andC, ( b(c)) 65 90◦ C°C. and (c) 90 ◦C.

3. Results and Discussion 3.1. Water Absorption Test The specimens were weighed prior to immersion in seawater at different temperatures. The specimens were taken out from the seawater tank and dried at room temperature for 24 h to determine the change in the mass. The percentage change in mass of specimen was calculated after drying as follows:

− (%) = × 100 (1)

where M (%) = mass change percentage, M2 = mass after drying (g), M1 = mass before immersion (g). Figure 2a shows the control specimen which is black in color with a clean surface. The effect of seawater and temperature was observed based on the crack on the surface of the specimen for different duration of time. In visual inspection, Figure 2b represents clear evidence of salt deposition on the surface of the samples immersed at 23 °C and 65 °C without any crack after 12 months of immersion in seawater. However, significant cracks were observed on the specimen immersed at 90 °C as shown in Figure 2c. The color of the specimen changed from black to gray for the specimen exposed at 90 °C whereas no change in color was observed for the specimen immersed at 23 °C and 65 °C. The change in the color of the specimen could be due to the micro-cracks which allow high-temperature seawater to penetrate through the layers of matrix and fiber results the removal of black pigment of the epoxy.

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3. Results and Discussion 3.1. Water Absorption Test The specimens were weighed prior to immersion in seawater at different temperatures. The specimens were taken out from the seawater tank and dried at room temperature for 24 h to determine the change in the mass. The percentage change in mass of specimen was calculated after drying as follows:

M − M M (%) = 2 1 × 100 (1) M1

where M (%) = mass change percentage, M2 = mass after drying (g), M1 = mass before immersion (g). Figure2a shows the control specimen which is black in color with a clean surface. The effect of seawater and temperature was observed based on the crack on the surface of the specimen for different duration of time. In visual inspection, Figure2b represents clear evidence of salt deposition on the surface of the samples immersed at 23 ◦C and 65 ◦C without any crack after 12 months of immersion in seawater. However, signiﬁcant cracks were observed on the specimen immersed at 90 ◦C as shown in Figure2c. The color of the specimen changed from black to gray for the specimen exposed at 90 ◦C whereas no change in color was observed for the specimen immersed at 23 ◦C and 65 ◦C. The change in the color of the specimen could be due to the micro-cracks which allow high-temperature Polymers 2021, 13, x FOR PEER REVIEW 5 of 21 seawater to penetrate through the layers of matrix and ﬁber results the removal of black pigment of the epoxy.

FigureFigure 2. E-glass/epoxyE-glass/epoxy specimen specimen conditioned conditioned at various at various temperatures temperatures in seawater in seawater (a) Control (a) Control sample,sample, ((bb)) 6565 ◦°CC and (c)) 9090 ◦°C.C.

TableTable2 2 and and FigureFigure3 3 show show thethe percentage ch changeange in in the the mass mass of of the the specimen specimen at atdif- differentferent conditioning conditioning periods. periods. The The specimen specimen mass mass increased increased with with immersion immersion time for thethe specimenspecimen conditioned at at 23 23 °C◦C and and 65 65 °C◦C but but it itreduced reduced for for the the specimen specimen immersed immersed at at90 90°C.◦ TheC. The seawater seawater absorption absorption or increase or increase in ma inss mass was was 0.9%, 0.9%, 1.3%, 1.3%, 1.7% 1.7% and and2.5% 2.5% after after 3 m, ◦ 36 m, 9 6 m m, and 9 m 12 and m of 12 conditioning m of conditioning at 23 °C at respectively 23 C respectively whereas whereas 2.8%, 2.9%, 2.8%, 4.1% 2.9%, and 4.1% 5% ◦ andfor seawater 5% for seawater immersion immersion at 65 °C. at Similarly, 65 C. Similarly, Ray [60] Ray observed [60] observed an increase an increase in water in absorp- water ◦ ◦ ◦ absorptiontion with the with increase the increase in temperature in temperature from 60 from °C to 60 70 C°C. to However, 70 C. However, at 90 °C, at the 90 massC, the of massthe specimen of the specimen dropped dropped by 1%, by3.6%, 1%, 5.9% 3.6%, and 5.9% 12% and after 12% immersion after immersion of 3 m, of 36 m,m, 69 m,m 9and m and12 m 12 respectively. m respectively. This This may may be bedue due to tothe the seawater seawater that that entered entered through through the crack andand degradeddegraded thethe ﬁber/matrixfiber/matrix bonding. Furthermore, at this temperature, the epoxy andand ﬁberfiber glassglass maymay notnot fuse together properly andand compositecomposite lostlost epoxyepoxy andand seawaterseawater occupiedoccupied thethe vacantvacant placeplace inin thethe specimen.specimen. Wang etet al.al. [[61]61] notednoted aa similarsimilar decreasedecrease inin thethe massmass ofof thethe compositecomposite atat elevatedelevated temperatures.temperatures.

Table 2. Percentage change in mass of E-glass/epoxy specimens.

Conditioning Duration Variation in Mass (%) (Months) 23 °C 65 °C 90 °C 3 0.9 ± 0.2 2.8 ± 0.3 1.07 ± 0.14 6 1.3 ± 0.4 2.9 ± 0.2 3.6 ± 0.9 9 1.7 ± 0.7 4.1 ± 0.6 5.9 ± 0.65 12 2.5 ± 0.5 5.0 ± 0.8 12.05 ± 1.67

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Table 2. Percentage change in mass of E-glass/epoxy specimens.

Conditioning Variation in Mass (%) Duration (Months) 23 ◦C 65 ◦C 90 ◦C 3 0.9 ± 0.2 2.8 ± 0.3 1.07 ± 0.14 Polymers 2021, 13, x FOR PEER REVIEW 6 1.3 ± 0.4 2.9 ± 0.26 of 3.6 21± 0.9

9 1.7 ± 0.7 4.1 ± 0.6 5.9 ± 0.65 12 2.5 ± 0.5 5.0 ± 0.8 12.05 ± 1.67

Figure 3. Effect of seawater conditioning on the mass of the specimen at various temperature. Figure 3. Effect of seawater conditioning on the mass of the specimen at various temperature. 3.2. Tensile Properties 3.2. Tensile Properties The tensile test was performed at room temperature (300 K) using an MTS universal The tensile testtesting was machineperformed (MTS at systemroom te corporation,mperature Eden(300 K) Prairie, using MN, an MTS USA). universal The tests were con- testing machine (MTSducted system at a crosshead corporation, speed ofEd 2en mm/min. Prairie, TableMN,3 USA). represents The thetests tensile were properties con- of the ducted at a crossheadE-glass/epoxy speed of composite2 mm/min. immersed Table 3 atrepresents 23 ◦C and the 65 ◦ tensileC for 12 properties months. The of tensile the strength E-glass/epoxy compositeof control immersed samples indicates at 23 °C the an tensiled 65 °C properties for 12 months. of the samples The tensile tested strength without immersion. of control samplesThese indicates results the were tensile used asproperties a reference of and the comparison samples tested with conditioned without immer- samples to deter- sion. These resultsmine were the used effect as of a exposurereference temperature and comparison and duration. with conditioned The tensile strengthsamples of to the control determine the effectspecimen of exposure was 798 temperature MPa. It was observed and duration. that the tensileThe tensile strength strength of the specimen of the reduced by 1% from 798 MPa to 790 MPa and 9% from 798 MPa to 726 MPa after an immersion of control specimen was 798 MPa. It was observed that the tensile strength of the specimen 12 months in seawater at 23 ◦C and 65 ◦C respectively. The tensile properties of specimens reduced by 1% fromconditioned 798 MPa in to seawater 790 MPa at 90and◦C 9% for 12from months 798 MPa are shown to 726 in MPa Table 4after. The an table im- indicates a mersion of 12 monthssharp in decrease seawater in tensile at 23 strength°C and 65 by °C 92.7% respectively. from 798 to The 57.3 MPatensile in 12properties months of seawater of specimens conditionedconditioning in seawater at 90 ◦C. at 90 °C for 12 months are shown in Table 4. The table indicates a sharp decrease in tensile strength by 92.7% from 798 to 57.3 MPa in 12 ◦ ◦ monthsTable of 3. seawaterTensile properties conditioning of E-glass/epoxy at 90 °C. composite immersed in seawater at 23 C and 65 C.

Immersion Time Tensile Strength (MPa) Tensile Modulus (GPa) Tensile Strain to Failure (%) Table 3. Tensile properties of E-glass/epoxy composite immersed in seawater at 23 °C and 65 °C. (Months) 23 ◦C 65 ◦C 23 ◦C 65 ◦C 23 ◦C 65 ◦C Immersion Time 0 (ControlTensile Sample) Strength (MPa) 798 ± 43 Tensile 798 Modulus± 43 (GPa) 37.1 ± 2.5 Tensile 37.1 ± 2.5Strain to 2.14 Failure± 0.02 (%) 2.14 ± 0.02 (Months) 233 °C 65 °C 796 ± 40 23 °C 783 ± 51 65 37.8 °C± 1.6 35.4 23 ±°C1.8 2.1 ± 650.2 °C 2.2 ± 0.0 6 794 ± 49 765 ± 17 35.1 ± 2.5 33.5 ± 0.6 2.26 ± 0.03 2.3 ± 0.2 0 (Control Sample) 7989 ± 43 798 ± 79343 ± 39 37.1 ± 7582.5± 28 37.1 36.3 ± 2.5± 1.9 2.14 33.7 ±± 0.022.3 2.22 2.14± 0.2 ± 0.02 2.24 ± 0.15 3 79612 ± 40 783 ± 79051 ± 38 37.8 ± 7261.6± 39 35.4 38.5 ± 1.8± 4.5 32.72.1 ±± 0.23.4 2.12 2.2± 0.21 ± 0.0 2.18 ± 0.1 6 794 ± 49 765 ± 17 35.1 ± 2.5 33.5 ± 0.6 2.26 ± 0.03 2.3 ± 0.2 9 793 ± 39 758 ± 28 36.3 ± 1.9 33.7 ± 2.3 2.22 ± 0.2 2.24 ± 0.15 12 790 ± 38 726 ± 39 38.5 ± 4.5 32.7 ± 3.4 2.12 ± 0.21 2.18 ± 0.1

Table 4. Tensile properties of the specimen immersed in seawater at 90 °C.

Immersion Time (Months) Tensile Strength (MPa) Tensile Strain to Failure (%) Tensile Modulus (GPa) 0 (Control sample) 798 ± 43 2.14 ± 0.02 37.1 ± 2.5 1 384 ± 55 1.27 ± 0.02 35.49 ± 2.13 2 368.7 ± 51 1.16 ± 0.023 34.07 ± 1.27 3 328.2 ± 19 0.88 ± 0.08 41.87 ± 2.48 4 279.9 ± 9 0.87 ± 0.07 42.79 ± 4.05 5 248 ± 27 0.75 ± 0.1 41.50 ± 2.81

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Table 4. Tensile properties of the specimen immersed in seawater at 90 ◦C.

Immersion Time Tensile Strength Tensile Strain to Tensile Modulus (Months) (MPa) Failure (%) (GPa) 0 (Control sample) 798 ± 43 2.14 ± 0.02 37.1 ± 2.5 Polymers 2021, 13, x FOR PEER REVIEW 7 of 21 1 384 ± 55 1.27 ± 0.02 35.49 ± 2.13 2 368.7 ± 51 1.16 ± 0.023 34.07 ± 1.27 3 328.2 ± 19 0.88 ± 0.08 41.87 ± 2.48 4 279.9 ± 9 0.87 ± 0.07 42.79 ± 4.05 6 5 215 ± 16 248 ± 27 0.51 ± 0.14 0.75 ± 0.1 41.50 40.43± ±2.81 1.23 7 6 165.6 ± 14 215 ± 16 0.31 ± 0.09 0.51 ± 0.14 40.43 43.35± ±1.23 4.33 8 7 151.9 ± 17 165.6 ± 14 0.40 ± 0.310.6 ± 0.09 43.35 42.1± ±4.33 3.9 ± ± ± 9 8 96.3 ± 26 151.9 17 0.3 ± 0.1 0.40 0.6 42.1 43.8 ±3.9 2.8 9 96.3 ± 26 0.3 ± 0.1 43.8 ± 2.8 10 10 74.7 ± 9 74.7 ± 9 0.27 ± 0.11 0.27 ± 0.11 47.38 47.38± ±2.8 2.8 11 11 60.5 ± 10 60.5 ± 10 0.15 ± 0.07 0.15 ± 0.07 47.18 47.18± ±1.13 1.13 12 12 57.3 ± 3.7 57.3 ± 3.7 0.14 ± 0.09 0.14 ± 0.09 46.9 46.9± ±4.7 4.7

FigureFigure4 shows4 shows nearly nearly a constant a constant line line for thefor specimenthe specimen immersed immersed at 23 ◦atC. 23 However, °C. However, a ◦ smalla small drop drop was was observed observed in thein the tensile tensile strength strength for thefor specimenthe specimen immersed immersed at 65 atC. 65 At °C. At ◦ 9090 °C,C, thethe tensiletensile strength strength dramatically dramatically reduced reduce tod 384to 384 MPa MPa in 1 in month 1 month which which is 48.12% is 48.12% of of thethe strengthstrength of of control control specimen. specimen. Furthermore, Furthermore, the the tensile tensile strength strength declined declined linearly linearly from from 384384 toto 57.357.3 MPa MPa in in the the remaining remaining period period of 11of 11 months months which which was was almost almost 14.9%. 14.9%.

FigureFigure 4.4.Effect Effect of of seawater seawater immersion immersion on on tensile tensile strength. strength.

FigureFigure5 5indicates indicates that that seawater seawater immersion immersion slightly slightly affected affected the elasticthe elastic modulus modulus of of the samples conditioned at 23 ◦C and 65 ◦C. The elastic modulus varied from 37.1 GPa the samples conditioned at 23 °C and 65 °C. The elastic modulus varied◦ from 37.1 ◦GPa to to38.5 38.5 GPa GPa and and 32.7 32.7 GPa GPa for for specimen specimen immersed immersed for for 12 12months months at at23 23°C andC and 65 65°C respec-C respectively. At an elevated temperature of 90 ◦C, elastic modus increased by 26.7% from tively. At an elevated temperature of 90 °C, elastic modus increased by 26.7% from 37.1 37.1 GPa to 46.9 GPa in the period of 12 months. The results indicate that the value of elastic GPa to 46.9 GPa in the period of 12 months. The results indicate that the value of elastic modulus varied between 34.07 GPa and 47.38 GPa in the entire conditioning duration. The elasticmodulus modulus varied of between the FRP 34.07 specimen GPa dependsand 47.38 on GP thea modulusin the entire of ﬁbers. conditioning The immersion duration. in The seawaterelastic modulus environment of the mayFRP notspecimen have any depends severe on effect the on modulus the durability of fibers. of theThe ﬁbres, immersion the in modulusseawater ofenvironment GFRP has not may suffered not have any obviousany severe change effect after on immersionthe durability [62]. of the fibres, the modulusThe failure of GFRP strain has increased not suffered progressively any obvious from 2.14%change to after 2.26% immersion after 6 months [62]. and then decreased to 2.12% after 12 months of immersion at 23 ◦C as shown in Figure6. However, immersion at 65 ◦C results in a slight increase in failure strain from 2.14% to 2.3% after 6 months, then reduced to 2.18% after 12 months. For immersion at 90 ◦C, the opposite

Polymers 2021, 13, x FOR PEER REVIEW 8 of 21

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pattern was observed for failure strain. The failure strain dropped sharply from 2.14 to Polymers 2021, 13, x FOR PEER REVIEW 8 of 21 1.27% in 1 month which is 59.2% of the control specimen. Thereafter, it declined gradually to 0.14% in the remaining 11 months with a comparatively slower rate.

Figure 5. Effect of seawater immersion on modulus of elasticity.

The failure strain increased progressively from 2.14% to 2.26% after 6 months and then decreased to 2.12% after 12 months of immersion at 23 °C as shown in Figure 6. How- ever, immersion at 65 °C results in a slight increase in failure strain from 2.14% to 2.3% after 6 months, then reduced to 2.18% after 12 months. For immersion at 90 °C, the opposite pattern was observed for failure strain. The failure strain dropped sharply from 2.14

to 1.27% in 1 month which is 59.2% of the control specimen. Thereafter, it declined grad- FigureFigureually to 5.5. Effect0.14%Effect of ofin seawaterseawater the remaining immersionimmersion 11 months onon modulusmodulus with ofof a elasticity.elasticity. comparatively slower rate.

FigureFigure 6.6. EffectEffect ofof seawaterseawater immersionimmersion onon failurefailure strainstrain (%).(%).

TheseThese datadata demonstratedemonstrate thatthat water water absorption absorption contributes contributes to to the the epoxy epoxy matrix matrix plasti- plas- cizationticization which which affects affects the the failure failure strain strain and tensileand tensile modulus. modulus. Overa Over duration a duration of 12 months of 12 ofmonths use, the of use, matrix the becomesmatrix beco stiffmes and stiff fragile, and fragile, allowing allowing failure failure strain strain to decrease to decrease and ten-and siletensile modulus modulus to increase to increase dramatically. dramatically. The The tensile tensile properties properties of the of the samples samples immersed immersed at ◦ 90at 90C °C signiﬁcantly significantly affected affected in in 12 12 months months of of immersion. immersion. The The failure failure strain strain of of the the samples samples

reducedreduced fromfrom 2.14%2.14% toto 0.14%0.14% andand tensiletensile modulusmodulus increasedincreased fromfrom 37.137.1 GPaGPa toto 46.946.9 GPa.GPa. MerahFigureMerah 6. etet Effect al.al. [[54]54 of] observedseawaterobserved immersion aa decreasedecrease on inin failure brittlebrittle strain fracturefracture (%). andand failurefailure strainstrain duedue toto prolongedprolonged immersionimmersion inin seawater.seawater. Further,TheseFurther, data thethe demonstrate effecteffect ofof temperaturetemperature that water in absorptionin seawaterseawater contributes conditioningconditioning to wasthewas epoxy examinedexamined matrix byby plas- thethe analogyticizationanalogy ofof which stress–strain stress–strain affects behavior thebehavior failure of theof strain samplethe sampleand immersed tensile immersed modulus. for 3 months,for 3Over months, 6 months,a duration 6 months, 9 months of 12 9 ◦ ◦ ◦ andmonths 12 months ofand use, 12 atthe months 23 matrixC, 65at beco 23C °C, andmes 65 90stiff °CC and and as shownfragile, 90 °C inasallowing Figureshown7 failure .in The Figure strength strain 7. Theto of decrease specimensstrength and of conditionedtensile modulus in seawater to increase at 23 dramatically.◦C decreased The by 0.25%tensile after properties 3 m, 0.5% of the after samples 6 months, immersed 0.75% afterat 90 9°C m significantly and by 1% affected after 12 in m 12 from months the of control immersion. specimen, The failure as shown strain in of Figure the samples7a–d, respectively.reduced from Figure 2.14%7 toa shows0.14% an thatd tensile the gradient modulus of theincreased specimen from immersed 37.1 GPa atto 2346.9◦ GPa.C is moreMerah as et compared al. [54] observed to the gradient a decrease of thein brittle control fracture specimen and demonstratingfailure strain due the to increase prolonged in immersion in seawater. Further, the effect of temperature in seawater conditioning was examined by the analogy of stress–strain behavior of the sample immersed for 3 months, 6 months, 9 months and 12 months at 23 °C, 65 °C and 90 °C as shown in Figure 7. The strength of

Polymers 2021, 13, x FOR PEER REVIEW 9 of 21

specimens conditioned in seawater at 23 °C decreased by 0.25% after 3 m, 0.5% after 6 months, 0.75% after 9 m and by 1% after 12 m from the control specimen, as shown in Polymers 2021, 13, 2182 Figure 7a–d, respectively. Figure 7a shows that the gradient of the specimen immersed8 of at 19 23 °C is more as compared to the gradient of the control specimen demonstrating the increase in tensile modulus by 1.9% from 37.1 GPa to 37.8 GPa after three months of conditioning.tensile modulus The slope by and 1.9% modulus from 37.1 decreased GPa to 37.8 a litt GPale by after 5.3% three after months 6 months, of conditioning. and 2.15% after The 9slope months and whereas modulus increased decreased again a little by by3.8% 5.3% in 12 after months 6 months, as indicated and 2.15% in Figure after 97b–d, months respectively.whereas increased The plots again also bydemonstrate 3.8% in 12 the months equivalent as indicated variation in Figurein failure7b–d, strain, respectively. first de- cliningThe plots by also 1.8% demonstrate then increasing the equivalent by 5.7% an variationd 3.7% inand failure again strain, decreasing ﬁrst declining by 0.9% byafter 1.8% 3 months,then increasing 6 months, by 5.7%9 months and 3.7% and and12 months again decreasing respectively. by 0.9%At 65 after °C, 3the months, strength 6 months, of the specimens9 months and reduced 12 months by 1.9% respectively. after 3 months, At 65 4.13%◦C, the after strength 6 months, of the 5% specimens after 9 months, reduced and by by1.9% 9% after after 3 12 months, months 4.13% as indicated after 6 months, in Figure 5% 7a–d, after respectively. 9 months, and The by slopes 9% after of the 12 monthsgraphs showas indicated the decrease in Figure in tensile7a–d, respectively.modulus by 11.8 The% slopes from of37.1 the MPa graphs to 32.7 show MPa the in decrease a duration in oftensile 12 months. modulus The by failure 11.8% strain from of 37.1 the specimen MPa to 32.7 slightly MPa increased in a duration by 1.8% of 12 after months. 12 months The offailure conditioning strain of theat 65 specimen °C as shown slightly in Figure increased 7d. byMaximum 1.8% after stain-at-failure 12 months of conditioningobserved after at 665 months◦C as shown by 7.5% in is Figure shown7d. in Maximum Figure 7b. stain-at-failure At 90 °C, the tensile observed strength after of 6 months the E-glass/epoxy by 7.5% is compositeshown in Figure declined7b. Atby 9058.9%◦C, after the tensile 3 months, strength 76.2% of theafter E-glass/epoxy 6 months, 86% composite after 9 m, declined and by 93.5%by 58.9% after after 12 3months months, as 76.2% shown after in 6Figure months, 7a– 86%d, respectively. after 9 m, and The by 93.5%slopes after of the 12 monthsgraphs indicateas shown that in Figurethe modulus7a–d, respectively. increased by The 26.7% slopes from of 37.1 the graphsMPa to indicate47 MPa thatin the the duration modulus of 12increased months. by 26.7% from 37.1 MPa to 47 MPa in the duration of 12 months.

Polymers 2021, 13, x FOR PEER REVIEW 10 of 21

(a) (b)

Figure 7. Stress–strainStress–strain behavior behavior of of specim specimenen immersed immersed in in seawater seawater for for (a ()a 3) months, 3 months, (b ()b 6) months, 6 months, (c) (9c )months 9 months and and (d) (12d) 12months. months.

3.3. SEM Analysis The microstructure of samples was analyzed using JEOL-JSM 7610F (JOEL Ltd., To- kyo, Japan) scanning electron microscope. Figure 8a represents the effect of seawater at 23 °C on the surface of the e-glass epoxy composite. The bonding between fiber and matrix is slightly affected without the presence of any crack. However, minor cracks and broken fiber were detected on the surface of the sample immersed at 65 °C as shown in Figure 8b. The crack size increased with the increase of immersion temperature. At the temperature of 90 °C, shown in Figure 8c, a significantly larger crack was observed which allows the seawater to penetrate the fiber layers. This penetration may cause the strength reduced by 93% in 12 months of immersion.

(a) (b) (c) Figure 8. Surface micrograph of specimen immersed for the duration of one-year at (a) 23 °C, (b) 65 °C and (c) 90 °C.

SEM images for the control sample compared with samples immersed at 23 °C and 65 °C mentioned in Figure 9a,b for the duration of 12 months. Rough fracture and chipping surface of fiber in Figure 9a indicates the ductile failure of the fiber. Shattered matrix and smooth fractured surface indicate brittle failure of the matrix. The microstructure of the fractured surface and fiber pullout represents slight degradation in fiber/matrix bonding. The composite indicated only a 1% decrease in tensile strength, a 3.77% increase in modulus due to swelling of the matrix and a 0.93% decrease in failure strain due to matrix plasticization.

Polymers 2021, 13, x FOR PEER REVIEW 10 of 21

(c) (d) Polymers 2021, 13, 2182 9 of 19 Figure 7. Stress–strain behavior of specimen immersed in seawater for (a) 3 months, (b) 6 months, (c) 9 months and (d) 12 months.

3.3. SEM SEM Analysis Analysis The microstructure ofof samplessamples was was analyzed analyzed using using JEOL-JSM JEOL-JSM 7610F 7610F (JOEL (JOEL Ltd., Ltd., Tokyo, To- kyo,Japan) Japan) scanning scanning electron electron microscope. microscope. Figure Figure8a represents 8a represents the effectthe effect of seawater of seawater at 23 at ◦23C °Con on the the surface surface of theof the e-glass e-glass epoxy epoxy composite. composite. The The bonding bonding between between fiber fiber and and matrix matrix is isslightly slightly affected affected without without the the presence presence of of any any crack. crack. However,However, minorminor crackscracks andand broken fiberfiber were detected on the surface of the sample immersed at 65 °C◦C as as shown shown in in Figure Figure 88b.b. The crack size increased with the increase of immersion temperature. At the temperature of 90 °C,◦C, shown in Figure 88c,c, aa significantlysignificantly largerlarger crackcrack waswas observedobserved whichwhich allowsallows thethe seawater to penetrate the fiber fiber layers. This This penetration penetration may cause the the strength strength reduced by 93% in 12 months of immersion.

(a) (b) (c)

Polymers 2021, 13, x FOR PEER REVIEW ◦ ◦ ◦ 11 of 21 Figure 8. Surface micrograph of specimen immersedimmersed forfor thethe durationduration ofof one-yearone-year atat ((aa)) 2323 °C,C, (b)) 6565 °CC and (c) 90 °C.C.

SEM images for the control sample compared with samples immersed at 23 ◦°CC and 65 ◦°CC mentioned inin FigureFigure9 a,b9a,b for for the the duration duration of 12of months.12 months. Rough Rough fracture fracture and and chipping chip- pingsurface surfaceThe of fibermicrograph of fiber in Figure in Figure 9ina indicatesFigure 9a indicates 9b the represents th ductilee ductile failure failure the of failure of the the fiber. fiber. surface Shattered Shattered of a matrix matrixspecimen immersed forand 12smooth smooth months fractured fractured at 65 surface surface °C. The indicate indicate smooth brittle brittle cross-sect failure failure of of theional the matrix. matrix. surface The The microstructureof microstructure the fibers ofindicates brittle fracturetheof the fractured fractured of thesurface surfacefiber. and Theand fiber presence fiber pullout pullout represents of representsepoxy slight resin slight degradation around degradation fibers in fiber/matrix in and fiber/matrix dimples bond- on the surface ofing.bonding. the The matrix composite The composite indicates indicated indicated ductile only onlya 1%failure. a decrease 1% decrease The in tensilefiber/matrix in tensile strength, strength, debonding a 3.77% a 3.77% increase increase and in potholing were modulusin modulus due due to swelling to swelling of the of matrix the matrix and a and 0.93% a 0.93% decrease decrease in failure in failure strain due strain to duematrix to observedplasticization.matrix plasticization. at the fractured surface which results in a 9% drop in tensile strength.

(a) (b) Figure 9. 9.Failure Failure surface surface of specimen of specimen immersed immersed for the duration for the ofduration 12 months of at 12 (a )months 23 ◦C, and at ((ba)) 23 °C, and (b) 65 ◦°C.C. The micrograph in Figure9b represents the failure surface of a specimen immersed for 12Figure months 10a–c at 65 ◦ C.show The smooththe SEM cross-sectional micrograph surface of E-glass/epoxy of the fibers indicates composit brittlee immersed at 90 °Cfracture for the of the duration fiber. The presenceof 3 months, of epoxy 6 resinmonths, around 9 fibersmonths and dimplesand 12 onmonths the surface respectively. The rough and corrugated surface of the fiber and dimpled matrix surface indicates the ductile failure of both fiber and matrix as shown in Figure 10a. The potholing and fiber/matrix debonding observed on the surface may results in a sharp decrease in the tensile strength of the composite as shown in Figure 4. With the increase of immersion time, resin flow and fiber/matrix debonding accelerated as shown in Figure 10b. The chipped-out fiber surface indicates the ductile fracture of the fiber whereas the smooth and mirror-like surface with matrix chunks indicates its brittle failure. The reaction between water and epoxy believed to cause a breakdown of the polymer’s molecular weight, leading to the fragile nature of the matrix. Water can also function as an anti-plasticizer, preventing polymer segments from moving and making the matrix more brittle [46]. After an immersion of 9 and 12 months, the fiber matrix bonding degraded to the lowest level and an absence of the resin was observed at the fractured surfaces of the composite as shown in Figure 10c,d. These SEM images support the observed decrease in the mass of the composite due to resin flow discussed in the water absorption test and shown in Figure 3. The degradation at the fiber/matrix interface involves a complex mechanism as the fiber/matrix interface is a heterogeneous area between the fiber and matrix [63–67]. Fiber-matrix interface damage for FRP composites is usually caused by debonding between fiber and resin which occurs mainly in two stages. The first stage is the chemical debonding due to chemical corrosion between fiber and resin, and the second stage is the poor interlacing between fibers and matrix owing to resin swelling through water absorption [68].

Polymers 2021, 13, 2182 10 of 19

of the matrix indicates ductile failure. The fiber/matrix debonding and potholing were observed at the fractured surface which results in a 9% drop in tensile strength. Figure 10a–c show the SEM micrograph of E-glass/epoxy composite immersed at 90 ◦C for the duration of 3 months, 6 months, 9 months and 12 months respectively. The rough and corrugated surface of the fiber and dimpled matrix surface indicates the ductile failure of both fiber and matrix as shown in Figure 10a. The potholing and fiber/matrix debonding observed on the surface may results in a sharp decrease in the tensile strength of the composite as shown in Figure4. With the increase of immersion time, resin flow and fiber/matrix debonding accelerated as shown in Figure 10b. The chipped-out fiber surface indicates the ductile fracture of the fiber whereas the smooth and mirror-like surface with matrix chunks indicates its brittle failure. The reaction between water and epoxy believed to cause a breakdown of the polymer’s molecular weight, leading to the fragile nature of the matrix. Water can also function as an anti-plasticizer, preventing polymer segments from moving and making the matrix more brittle [46]. After an immersion of 9 and 12 months, the fiber matrix bonding degraded to the lowest level and an absence of the resin was observed at the fractured surfaces of the composite as shown in Figure 10c,d. These SEM images support the observed decrease in the mass of the composite due to resin flow discussed in the water absorption test and shown in Figure3. The degradation at the fiber/matrix interface involves a complex mechanism as the fiber/matrix interface is a heterogeneous area between the fiber and matrix [63–67]. Fiber-matrix interface damage for FRP composites is usually caused by debonding between fiber and resin which occurs mainly in two stages. The first stage is the chemical debonding due to chemical corrosion Polymers 2021, 13, x FOR PEER REVIEW 12 of 21 between fiber and resin, and the second stage is the poor interlacing between fibers and matrix owing to resin swelling through water absorption [68].

(a) (b)

FigureFigure 10. 10.Failure Failure surface surface of of specimen specimen immersed immersed in seain sea water water at 90 at◦ C(90 a°C) 3 (a months,) 3 months, (b) 6 ( months,b) 6 months, ((cc)) 9 9 months months and and (d ()d 12) 12 months. months.

3.4. Differential Scanning Calorimetry (DSC) Test The differential scanning calorimetry (DSC) for the E-glass/epoxy composite was conducted on samples immersed at different temperatures and exposure times. This test was carried out using a TA-Instruments DSC Q200 device (TA-Instruments, New Castle, DE, USA). The test was performed in an inert environment using nitrogen gas. The experiment was run between 25 °C and 250 °C with a heating rate of 10.0 °C/min. Three samples for each condition were considered to determine the glass transition temperature. Figure 11 represents the DSC curves for the control sample and specimens immersed at 23 °C, 65 °C and 90 °C for 12 months. The glass transition temperatures (Tg) of the samples immersed at 23 °C and 65 °C found to be close to the Tg of the control sample. The glass transition temperature for the control specimen and specimen immersed at 23 °C and 65 °C observed to be 118 °C, 117 °C and 112 °C, respectively, indicating that as exposure temperature increases, the Tg decreases.

Polymers 2021, 13, 2182 11 of 19

3.4. Differential Scanning Calorimetry (DSC) Test The differential scanning calorimetry (DSC) for the E-glass/epoxy composite was conducted on samples immersed at different temperatures and exposure times. This test was carried out using a TA-Instruments DSC Q200 device (TA-Instruments, New Castle, DE, USA). The test was performed in an inert environment using nitrogen gas. The experiment was run between 25 ◦C and 250 ◦C with a heating rate of 10.0 ◦C/min. Three samples for each condition were considered to determine the glass transition temperature. Figure 11 represents the DSC curves for the control sample and specimens immersed at 23 ◦C, 65 ◦C and 90 ◦C for 12 months. The glass transition temperatures (Tg) of the samples immersed at 23 ◦C and 65 ◦C found to be close to the Tg of the control sample. The glass transition temperature for the control specimen and specimen immersed at 23 ◦C and 65 ◦C observed Polymers 2021, 13, x FOR PEER REVIEW ◦ ◦ ◦ 13 of 21 to be 118 C, 117 C and 112 C, respectively, indicating that as exposure temperature increases, the Tg decreases.

FigureFigure 11.11.DSC DSC curves curves for for E-glass/epoxy E-glass/epoxy control control sample sample and conditionedand conditioned samples. samples. This is an indication of the associated effect of immersion on the physical and chemical This is an indication of the associated effect of immersion on the physical and chem- properties of the composite. It was noted that the tensile strength reduced by 1% and 9% icalin one properties year for theof the exposure composite. temperature It was ofnot 23ed◦C that and the 65 ◦tensileC, respectively. strength Furthermore,reduced by 1% and 9%SEM in micrographs one year for and the FTIR exposure analysis temperature support the observedof 23 °C and slight 65 degradation. °C, respectively. On the Furthermore, other SEMside, themicrographs Tg of the composite and FTIR was analysis signiﬁcantly support reduced the to observed 42.11 ◦C after slight seawater degradation. aging at On the other90 ◦C forside, 12 the months. Tg of The the value composite of Tg was was observed significantly to be 69.8 reduced◦C, 51.55 to 42.11◦C, 46.57 °C ◦afterC and seawater ◦ aging42.11 Cat for90 the°C durationfor 12 months. of 3 months, The 6value months, of 9Tg months was observed and 12 months, to be respectively.69.8 °C, 51.55 This °C, 46.57 °Csigniﬁcant and 42.11 shift °C in for Tg the is because duration of theof 3 combined months, effect 6 months, of high 9 temperaturemonths and and 12 seawatermonths, respectively.aging. TheThis degradation significant in shift the epoxy in Tg composite is becaus owese of tothe the combined existence ofeffect hydrophilic of high groups temperature which form weak hydrogen bonds by reacting with water molecules during immersion and seawater aging. The degradation in the epoxy composite owes to the existence of hy- and swelling of the composite at 90 ◦C[69]. This demonstrates that the moisture uptake drophilicacts as a plasticizer groups andwhich decreases form weak Tg. The hydrogen reduction inbonds Tg represents by reacting thermal with degradation water molecules duringand loss immersion in mechanical and properties swelling of which the composite limits the service at 90 °C temperature [69]. This ofdemonstrates the polymer. that the moistureThe mechanisms uptake of acts the longas a chainplasticizer of the polymer and dec mayreases start Tg. to isolateThe reduction at high-temperature in Tg represents thermalimmersion degradation and react with and each loss other in mechanical to alter the propertiesproperties of which the polymer limits [the70]. service temperature of the polymer. The mechanisms of the long chain of the polymer may start to isolate 3.5. FTIR Results at high-temperature immersion and react with each other to alter the properties of the polymerThe Fourier[70]. transform infrared spectroscopy (FTIR) was performed on Perkin Elmer Spectrum 100 FTIR spectrometer (PerkinElmer Life and Analytical Sciences, Shelton, CT, USA) at room temperature in the transmission mode. FTIR spectra were logged in between 3.5. FTIR Results 600 cm−1 and 4000 cm−1 at a resolution of 2 cm−1 with 10 scans. Before testing the The Fourier transform infrared spectroscopy (FTIR) was performed on Perkin Elmer Spectrum 100 FTIR spectrometer (PerkinElmer Life and Analytical Sciences, Shelton, CT, USA) at room temperature in the transmission mode. FTIR spectra were logged in between 600 cm−1 and 4000 cm−1 at a resolution of 2 cm−1 with 10 scans. Before testing the samples, Background spectra were taken in the empty chamber to eliminate the influence of moisture and CO2 in air. Figure 12 presents a comparison of typical spectra for E- glass/epoxy composite immersed at 23 °C, 65 °C and 90 °C for 12 months and compared with control sample. The seawater exposure will lead to the presence of equivalent FTIR bands on the residue spectrum due to the leaching of functional groups from the resin of the composite. The allocation of the representative absorbent bands is given in Table 5 for the samples and their allocated functional groups. The O–H stretching band is observed above 3000 cm−1 wavenumbers.

Polymers 2021, 13, 2182 12 of 19

samples, Background spectra were taken in the empty chamber to eliminate the inﬂuence of moisture and CO2 in air. Figure 12 presents a comparison of typical spectra for E- glass/epoxy composite immersed at 23 ◦C, 65 ◦C and 90 ◦C for 12 months and compared with control sample. The seawater exposure will lead to the presence of equivalent FTIR bands on the residue spectrum due to the leaching of functional groups from the resin of the composite. The allocation of the representative absorbent bands is given in Table5 for Polymers 2021, 13, x FOR PEER REVIEW 14 of 21 the samples and their allocated functional groups. The O–H stretching band is observed above 3000 cm−1 wavenumbers.

FigureFigure 12. 12.FTIR FTIR spectrum spectrum of E-glass/epoxyof E-glass/epoxy control control sample sample and conditioned and conditioned samples. samples.

TableTable 5. 5.FTIR FTIR bands bands observed observed in E-glass/epoxy in E-glass/epoxy specimens. specimens.

BandsBands (cm− (cm1) −1) Assignment Assignment 3400 3400 Stretching Stretching vibration vibration O=H O=H ~2930 ~2930and ~2900 and ~2900 C–H C–H group group stretching stretching band band ~1732 C–O non-conjugate ester stretching ~1732~1610 C–O non-conjugat stretching band ofe ester C=C (alkene)stretching ~1610~1509 stretching C=C band (aromatic of C=C nucleus) (alkene) ~1245 Asymmetric C–O– Φ stretch ~15091182 C=C C–O aromatic(aromatic ring nucleus) stretching ~1245~1040 Asymmetric Symmetric C–O– Φ Φstretch stretch 1182 ~827 Out C–O of planearomatic bending ring of C–Hstretching (benzene) ~1040 Symmetric C–O– Φ stretch −1 The difference~827 in intensity of O–H stretching Out of vibrationplane bending at 3400 of cm C–His (benzene) due to the seawater immersion of the specimen [71,72]. The next band is located between 2930 cm−1 and 2900 cm−1 and is attributed to the stretching band of the C–H group of epoxies [71,72]. The difference in intensity of O–H stretching vibration at 3400 cm−1 is due to the sea- The stretching of C–O non-conjugate ester was detected at 1732 cm−1 [73]. The existence of −1 bandswater at immersion 1509 cm−1 andof the 1610 specimen cm−1 are [71,72]. allotted The for C=C next stretching band is inlocated aromatics between and alkenes 2930 cm and respectively2900 cm−1 and [72, 74is ,attributed75]. Symmetric to the and stretching asymmetric band stretching of the vibration C–H group of C–O– of epoxiesΦ observed [71,72]. The atstretching 1040 cm− 1ofand C–O 1245 non-conjugate cm−1 wavenumber ester respectively was detected [71, 73at]. 1732 The bandcm−1 at[73]. 1182 The cm− existence1 is of duebands to theat 1509 vibration cm−1 characteristics and 1610 cm of−1 are C–O allotted in an aromatic for C=C ring stretching [76] whereas in aromatics C–H bending and alkenes − inrespectively benzene ring [72,74,75]. was detected Symmetric at 827 cm and1 [72 asymmetric]. The immersion stretching of specimen vibration in seawater of C–O–Φ ob- −1 producesserved at a 1040 band cm at−1 ~840 and cm1245 cmwhich−1 wavenumber indicates stretching respectively vibration [71,73]. of the The ether band group at 1182 cm−1 is due to the vibration characteristics of C–O in an aromatic ring [76] whereas C–H bending in benzene ring was detected at 827 cm−1 [72]. The immersion of specimen in seawater produces a band at ~840 cm−1 which indicates stretching vibration of the ether group as trapped moisture in specimen includes hydrogen bond with the C-O-C groups [72]. An- other band that appeared at ~1125 cm−1 could be ascribed to the alteration in the vibrational C-OH band leading to the formation of hydrogen bond (C–O–H----OH2) during hydration [72]. The presence of these bands confirms the leaching of the E-glass/epoxy matrix into the seawater during immersion. In the FTIR spectrum, two peaks at 2296.3 cm−1 and 2353.2 cm−1 appeared for the control sample. This indicates the existence of unreacted –N=C=O groups [77] in E- glass/epoxy samples. These two peaks become weak after immersion of 12 months at 23 °C and 65 °C and disappeared at 90 °C. The vanishing of the two peaks at 2296.3 cm−1 and

Polymers 2021, 13, 2182 13 of 19

as trapped moisture in specimen includes hydrogen bond with the C-O-C groups [72]. Another band that appeared at ~1125 cm−1 could be ascribed to the alteration in the vibrational C-OH band leading to the formation of hydrogen bond (C–O–H—-OH2) during hydration [72]. The presence of these bands conﬁrms the leaching of the E-glass/epoxy matrix into the seawater during immersion. In the FTIR spectrum, two peaks at 2296.3 cm−1 and 2353.2 cm−1 appeared for the control sample. This indicates the existence of unreacted –N=C=O groups [77] in E- glass/epoxy samples. These two peaks become weak after immersion of 12 months at 23 ◦C and 65 ◦C and disappeared at 90 ◦C. The vanishing of the two peaks at 2296.3 cm−1 and 2353.2 cm−1 after immersion for 12 months at 90 ◦C is due to the reaction between the water molecules and unreacted –N=C=O groups, which can be presented shown as

R − N = C = O + H2O → R − NH2 + CO2 ↑ (2)

Therefore, the mass loss of the E-glass epoxy composite specimens during immersion ◦ at 90 C can be described as the release of CO2 shown in Equation (2). At this immersion temperature, the sustained release of CO2 is responsible for the increase of mass loss of the composite [67,78,79].

4. Prediction Several researchers utilized the Arrhenius method for the prediction of the long-term behavior of FRP composites [48,80,81]. A fundamental hypothesis was anticipated that the single dominant degradation mechanism does not alter irrespective of temperature and exposure time [33], but the damage resistance reduces with the exposure temperatures. The equation for the degradation rate is mentioned below [82];

k = A e(−Ea/RT) (3)

where k is the rate of degradation (1/time), A is the constant of material in the degradation process, Ea is the activation energy (kJ/mol), R is the universal gas constant (8.314 Jmol−1K−1), T is the temperature (K). Equation (3) can be modiﬁed as:

1 1 ( Ea ) = e RT (4) k A 1 E 1 ln = a − ln(A) (5) k R T The inverse of k indicates the time taken to achieve a given value for a material property. The exponential degradation model [83] was utilized as the ﬁber-matrix interfacial delamination took place during the seawater immersion of E-glass/epoxy composite (Figure 13). This model can be expressed as

( −t ) Y = 100e τ (6)

where Y is the tensile strength retention (%), τ is a constant and t is the immersion time (month). the value of correlation coefﬁcient (R2) and τ of E-glass/epoxy composite are mentioned in Table6 by ﬁtting Figure 13 using Equation (6).

Table 6. Coefﬁcients of regression equation.

Temperature τ R2 23 1250 1 65 142.86 0.9544 90 4.95 0.966 Polymers 2021, 13, x FOR PEER REVIEW 15 of 21

2353.2 cm−1 after immersion for 12 months at 90 °C is due to the reaction between the water molecules and unreacted –N=C=O groups, which can be presented shown as

R−N=C=O+HO→R−NH +CO ↑ (2) Therefore, the mass loss of the E-glass epoxy composite specimens during immersion at 90 °C can be described as the release of CO2 shown in Equation (2). At this immersion temperature, the sustained release of CO2 is responsible for the increase of mass loss of the composite [67,78,79].

= ( ) (3) where k is the rate of degradation (1/time), A is the constant of material in the degradation process, Ea is the activation energy (kJ/mol), R is the universal gas constant (8.314 Jmol- 1K−1), T is the temperature (K). Equation (3) can be modified as:

1 1 = (4)

1 1 ln = −ln () (5) The inverse of k indicates the time taken to achieve a given value for a material property. The exponential degradation model [83] was utilized as the fiber-matrix interfacial delamination took place during the seawater immersion of E-glass/epoxy composite (Fig- ure 13). This model can be expressed as = 100 (6)

Polymers 2021, 13, 2182 where Y is the tensile strength retention (%), τ is a constant and t is the immersion 14 of 19 time (month). the value of correlation coefficient (R2) and τ of E-glass/epoxy composite are mentioned in Table 6 by fitting Figure 13 using Equation (6).

Figure 13. Tensile strength retention of E-glass epoxy samples exposed to seawater at 23 ◦C, 65 ◦C, and 90 ◦C.

A different approach based on a time-shift factor (TSF) was used by Dejke [49]. The time-shift is deﬁned as the ratio of times (t1 and t2) required to determine a certain level of

decrease in the mechanical property at two different temperatures (T1 and T2). According to Equation (7), the time required to obtain a certain level of decrease in mechanical property is the inverse proportion of the reaction rate K.

− Ea (1 − Y) T R Ea 1 1 t1 /K1 K2 Ae 2 ( − ) TSF = = = = = e R T2 T1 (7) t (1 − Y)/K K − Ea 2 2 1 Ae T1R This approach also depends on the Arrhenius equation. The time shift is an effect of accelerated aging due to the increment in temperature from T2 (lower temperature) to T1 (higher temperature) [84]. The TSF was calculated, using Equation (7), shown in Table7 for 65 ◦C and 90 ◦C.

Table 7. Value of time-shift factor and activation energy.

Temperature TSF Ea/R 65 8.7 5155.713 90 456 9731.482

The value of Ea/R can also be calculated using the time-shift approach by plotting the TSF versus the temperature (in ◦C) graph as shown in Figure 14 and fitted by regression analysis so that the fit is given by Equation (7). The values of Ea/R are presented in Table7. The durability of composite with a reference temperature of 23 ◦C calculated by time- shift from 23 ◦C to 65 ◦C and 23 ◦C to 90 ◦C. The TSF values calculated from Equation (7) ◦ ◦ using temperatures 23 C and 90 C is 456 and the time t2 to reach 57% of the strength at ◦ ◦ 90 C is 12 months, then the time t1 to reach 7.2% strength at 23 C is 456 × 12 = 5472 months. The extrapolations to predict the long-term tensile strength retention at 23 ◦C from the strength retention at 65 ◦C and 90 ◦C based on the time-shift approach for E-glass/epoxy are shown in Figure 15. The discussed method can be utilized to predict the durability of any structure under seawater at any temperature. In contrast, E-glass/epoxy possesses a long-term performance in tensile strength under seawater immersion at 23 ◦C. However, at an accelerated temperature of 90 ◦C, the tensile and chemical properties of the composite degraded significantly in a short period of immersion which restricts the application of the composite at high-temperature immersion. Polymers 2021, 13, x FOR PEER REVIEW 16 of 21

Figure 13. Tensile strength retention of E-glass epoxy samples exposed to seawater at 23 °C, 65 °C, and 90 °C.

Table 6. Coefficients of regression equation.

Temperature τ R2 23 1250 1 65 142.86 0.9544 90 4.95 0.966

A different approach based on a time-shift factor (TSF) was used by Dejke [49]. The time-shift is defined as the ratio of times (t1 and t2) required to determine a certain level of decrease in the mechanical property at two different temperatures (T1 and T2). According to Equation (7), the time required to obtain a certain level of decrease in mechanical property is the inverse proportion of the reaction rate K.

(1 − ) t K Ae TSF = = = = =e (7) t (1 − ) K Ae This approach also depends on the Arrhenius equation. The time shift is an effect of accelerated aging due to the increment in temperature from T2 (lower temperature) to T1 (higher temperature) [84]. The TSF was calculated, using Equation (7), shown in Table 7 for 65 °C and 90 °C.

Table 7. Value of time-shift factor and activation energy.

Temperature TSF Ea/R 65 8.7 5155.713 90 456 9731.482

The value of Ea/R can also be calculated using the time-shift approach by plotting the Polymers 2021, 13, 2182 TSF versus the temperature (in °C) graph as shown in Figure 14 and fitted by regression15 of 19 analysis so that the fit is given by Equation (7). The values of Ea/R are presented in Table 7.

Polymers 2021, 13, x FOR PEER REVIEW 17 of 21

The durability of composite with a reference temperature of 23 °C calculated by time- shift from 23 °C to 65 °C and 23 °C to 90 °C. The TSF values calculated from Equation (7) using temperatures 23 °C and 90 °C is 456 and the time t2 to reach 57% of the strength at 90 °C is 12 months, then the time t1 to reach 7.2% strength at 23 °C is 456 × 12 = 5472 months. The extrapolations to predict the long-term tensile strength retention at 23 °C from the strength retention at 65 °C and 90 °C based on the time-shift approach for E- glass/epoxy are shown in Figure 15. The discussed method can be utilized to predict the durability of any structure under seawater at any temperature. In contrast, E-glass/epoxy possesses a long-term performance in tensile strength under seawater immersion at 23 °C. However, at an accelerated temperature of 90 °C, the tensile and chemical properties of the composite degraded significantly in a short period of immersion which restricts the applicationFigureFigure 14. 14. TheThe of TSF TSFthe values valuescomposite versus versus attemperatur temperature high-temperaturee for for E-glass E-glass immersion.epoxy epoxy composite. composite.

FigureFigure 15. 15. DurabilityDurability prediction prediction of of E-glass epoxy composite immersed in seawater at 23 °C.◦C.

5.5. Conclusions Conclusions In this study, E-glass/epoxy composites were conditioned at different exposure tem- In this study,◦ E-glass/epoxy◦ ◦ composites were conditioned at different exposure temperatures (23 °C,C, 65 °CC and and 90 90 °C)C) in in Gulf Gulf seawater for a period of 12 months. The water absorption, tensile properties and failure analysis were conducted and the durability of absorption, tensile properties and failure analysis were conducted and the durability of the composite predicted using the TSF approach. The outcomes of the investigation are the composite predicted using the TSF approach. The outcomes of the investigation are summarized below: summarized below: (1) The mass of the specimen increased by 2.5% and 5% after the immersion of 12 months (1) The mass of the specimen increased by 2.5% and 5% after the immersion of 12 months at 23 ◦C and 65 ◦C respectively. An opposite tendency was observed for the specimen at 23 °C and 65 °C respectively. An opposite tendency was observed for the specimen immersed at 90 ◦C in which the mass of the sample reduced by 12.7% for the same immersed at 90 °C in which the mass of the sample reduced by 12.7% for the same immersion period. immersion period. (2) The tensile strength reduced by 1% and 9% after immersion of 12 months at 23 ◦C (2) The tensile strength reduced by 1% and 9% after immersion of 12 months at 23 °C and 65 ◦C respectively. The durability of the composite was signiﬁcantly affected at and 65 °C respectively. The durability of the composite was significantly affected at at 90 ◦C and tensile strength was reduced to 48.4% in 1 month. at 90 °C and tensile strength was reduced to 48.4% in 1 month. (3) Slight variation in the tensile modulus observed for specimen immersed at 23 ◦C

(3) Slightand 65 variation◦C where in the it increasedtensile modulus signiﬁcantly observed at for 90 specimen◦C. However, immersed the failure at 23 °C strain and 65slightly °C where increased it increased for specimens significantly at 23 ◦ Cat and90 °C. 65 ◦However,C but it decreased the failure drastically strain slightly for the increasedimmersion for at specimens 90 ◦C. at 23 °C and 65 °C but it decreased drastically for the immersion at 90 °C. (4) SEM micrographs indicate fiber/matrix debonding, potholing, fiber pull-out and matrix cracking which indicates deterioration in the tensile properties of the composite. The deterioration mainly owes to breakage of chemical bonding between fiber and resin due to chemical corrosion, and poor interlocking between fibers and resin due to resin swelling through water absorption especially at 90 °C immersion, mainly from the hydrolysis of resin, which was also evidenced by the DSC and FTIR results.

Polymers 2021, 13, 2182 16 of 19

(4) SEM micrographs indicate fiber/matrix debonding, potholing, fiber pull-out and matrix cracking which indicates deterioration in the tensile properties of the composite. The deterioration mainly owes to breakage of chemical bonding between fiber and resin due to chemical corrosion, and poor interlocking between fibers and resin due to resin swelling through water absorption especially at 90 ◦C immersion, mainly from the hydrolysis of resin, which was also evidenced by the DSC and FTIR results. (5) A prediction approach based on a time-shift factor (TSF) was used which utilizes the accelerated temperature testing results to build a model for the long-term prediction at room temperature. This model predicted that the tensile strength retention of E- glass/Epoxy composite will be reduced to 7% 450 years after immersion in seawater at 23 ◦C. Lastly, the activation energy for the degradation of the composite was calculated. It was 5155.713 and 9731.482 for composite immersed at 65 ◦C and 90 ◦C respectively. It can also be concluded that the E-glass/epoxy composite showed outstanding performance in seawater immersion up to 65 ◦C. However, the rate of degradation was significantly high for immersion at 90 ◦C and the material almost lost its durability. This is an indicative measure that the durability of the E-glass/epoxy composite is uncertain above 65 ◦C which may limit its utilization for high-temperature applications.

Author Contributions: Conceptualization, A.H.I. and A.-H.I.M.; methodology, A.H.I. and A.-H.I.M.; formal analysis, A.H.I.; investigation, A.H.I. and A.-H.I.M.; resources, A.-H.I.M. and B.M.A.-M.; data curation, A.H.I.; writing—original draft preparation, A.H.I.; writing—review and editing, A.-H.I.M., B.M.A.-M. and B.S.; visualization, A.H.I. and A.-H.I.M.; supervision, A.-H.I.M. and B.M.A.-M.; project administration, A.-H.I.M., B.M.A.-M., and B.S.; funding acquisition, A.-H.I.M. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the UPAR research program at the United Arab Emirates University, UAE [grant number 31N225]. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: All data and models used during the study appear in the submitted article. Acknowledgments: The authors would also like to acknowledge the UAEU and WSU for providing the research facilities. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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